In general, commercially pure titanium uses titanium sponge or titanium scrap as a raw material. It is melted by non-consumable electrode arc remelting, electron-beam remelting, plasma arc remelting, or the like into a titanium ingot (titanium cast product). Non-consumable arc remelting uses titanium sponge pressed into a briquette as an electrode, and causes arc discharge between the electrode and a mold to melt the electrode itself and cast it into the mold, thereby obtaining an ingot. Therefore, uniform discharge between the electrode and the mold is necessary, which limits the shape of the mold to a cylindrical shape; accordingly, the shape of the ingot after casting is a cylindrical shape. On the other hand, electron-beam remelting and plasma arc remelting, which use electron beams and plasma arc, respectively, differ in melting method, but both the methods pour molten titanium melted on a hearth into a mold, and this allows free selection of the shape of the mold; thus, it is possible to produce ingots with various shapes, such as a rectangular shape and a billet shape, as well as a cylindrical shape.
In the current titanium material production process, after this, a hot working process, such as slabing or forging, which is called an ingot breakdown process, is carried out and then hot rolling is performed; the breakdown process is necessary. However, according to the shapes, it is considered that the breakdown process can be omitted in producing a sheet material for a rectangular ingot (slab) and in producing a bar or a wire rod for a cylindrical ingot and a billet, and a technology of performing hot rolling without the breakdown process has been under study. If this technology is established, it can be expected that cost will be improved by omission of a process and an enhancement in yield.
However, a titanium cast product produced by electron-beam remelting or plasma arc remelting is as-cast and therefore comprises coarse grains with sizes as large as several tens of millimeters. In regard to such a titanium cast product, when hot rolling is performed without a breakdown process, because of the coarse grains, the influence of deformation anisotropy in a grain and between crystal grains causes surface unevenness, leading to surface defects. In order to remove surface defects that occur in hot rolling, it is necessary to increase the amount of pickling of the surface of a hot-rolled material in a pickling process, which is the following process, and accordingly yield is worsened and may result in an increase in cost.
Accordingly, for a titanium ingot produced by electron-beam remelting or plasma arc remelting, while it is expected that cost will be improved by omission of a breakdown process carried out by slabing, forging, or the like, there is a concern that an increase in surface defects may cause an increase in cost. This has inhibited practical utilization of a titanium cast product obtained without a breakdown process.
Patent Literature 1 discloses a method that provides an excellent casting surface and can improve surface defects after hot rolling even when an ingot breakdown process is omitted in the following case: in a cross-sectional microstructure of a titanium slab produced in an electron beam remelting furnace and extracted directly from a mold, an angle θ formed by the solidification direction from the surface layer toward the interior and the casting direction of the slab is in the range of 45 to 90°, or in the crystal orientation distribution of the surface layer, an angle formed by the c-axis of hcp and the normal to the slab surface layer is in the range of 35 to 90°. That is, controlling the shape and crystal orientation of crystal grains of the surface suppresses occurrence of defects due to coarse crystal grains.
In Patent Literature 2, as a method for directly performing hot rolling without an ingot breakdown process for a titanium material, the surface layer at a surface corresponding to a surface to be rolled is subjected to melting and resolidification by high-frequency induction heating, arc heating, plasma heating, electron-beam heating, laser heating, and the like; thus, a portion from the surface layer to a depth of 1 mm or more undergoes grain refining. This slab surface layer is quenched and solidified to have fine and irregular crystal orientation distribution, which prevents occurrence of surface defects.